The present invention relates to an exhaust gas purifying device for a diesel engine. More particularly, it relates to a combustion device such as a regenerative burner for a car which performs heating, burning and removing fine particles, in exhaust gas, deposited on a filter to eliminate the clogging of the filter.
The particles contained in exhaust gas discharged from cars such as diesel engines are harmful to a human body and lawful restrictions on the amount of fine particles dishcarged from the diesel engines have been considered or executed in many countries.
Various measures have been proposed to reduce the amount of discharged fine particles. An effective way is, for example, placing a filter in the exhausting system of a diesel engine to catch fine particles thereon and then the filter is heated by a high temperature combustion gas ejected from a regenerative burner to burn the fine particles deposited on the filter, thereafter they are removed.
Problems imposed on the regenerative burner is that reliable, instant ignition should be attained as well as stable combustion and a satisfied combustion under a high burden should be attained in addition to miniaturization required for restriction of car space even under conditions of broad change in temperature and vibration load which are inherent in a car.
FIG. 1 is a sectional view showing an example of a conventional regenerative burner in which the reference numeral 1 designates a high pressure atomizing type or a twin fluid atomizing type nozzle, the numeral 2 refers to a whirling air supplying passage, the numeral 3 indicates upper whirling-air apertures, the numeral 3a denotes upper whirling-air fed from the apertures 3, the numeral 4 designates lower whirling-air apertures, the numeral 4a indicates lower whirling-air fed from the apertures 4, the numeral 5 indicates a pair of ignition plugs, the numeral 6 represents a combustion tube, the numeral 7 refers to flame produced by firing atomized fuel and the numeral 8 denotes a part of an exhaust pipe system of an engine.
In the device having the construction as above-mentioned, fuel or a primary fuel mixture atomized and ejected from the nozzle 1 is mixed with the upper whirling-air 3a fed through the upper whirling-air apertures 3, then is fired by means of the ignition plugs 5 to form the flame 7 in the combustion tube 6 through the agency of the lower whirling-air 4a fed from the lower whirling-air apertures, and thereafter combustion gas is led to a filter through the exhaust pipe 8.
The structure of the regenerative burner, however, has disadvantages of wearing of electrodes because the ignition plugs are always in contact with the flame 7 and of causing deterioration in firing property due to contamination of the ignition plugs with soot, tar and so on. There are further disadvantages that since the growth of the core of flame produced by firing is inhibited due to the cooling function of whirling-air which disturbs ambience around the electrodes of the ignition plugs 5, time required for totally firing, namely delay time in firing is prolonged and variation in the delay time of firing when the ratio of fuel to air is changed is also large.
In the burner having the construction as above-mentioned, although an attempt has been made to obtain a stabilized ignition surface by forming whirling-air streams, there takes place the drawback as follows. Since a force acts on the flame 7 in the directions of its center axis and the nozzle, the ignition surface is produced at a position very close to the nozzle and the upper whirling-air 3a causes a part of the flame 7 to contact with the nozzle to thereby deposit soot on the nozzle surface. Further, there takes place a temperature rise in the nozzle surface by thermal transmission and thermal radiation from the flame to thereby cause the fuel, remaining non-burned, deposited on the nozzle surface to form tar. The contamination of the nozzle by the accumulation of the soot, tar and other materials rapidly changes the area of the ejection opening of the nozzle and in the extreme case, the ejection opening may be clogged. Thus, in the conventional regenerative burner, it has been difficult to obtain a stable combustion through its life time due to, especially, occurence of the vapor lock of fuel in the nozzle which is caused by the temperature rise in the nozzle.